1. Field of the Invention
This invention relates to phased array antenna phase shifters and more particularly to a select-line, base-3 phase shifter using micro electro mechanical system (MEMS) technology.
2. Description of the Related Art
Phased array antennas comprise multiple radiating elements whose transmission signals combine during operation to yield superior results over a single element antenna. The transmission signals of the multiple radiating elements are typically phased and summed during transmission to yield a synthesized transmission beam. The resulting transmission beam contains the combined power of all the multiple radiating elements and provides a single agile and inertialess beam that is useful in many applications, including radar and communication.
During operation of a phased array antenna, it is useful to steer the phased array transmission beam. Often the steering of the transmission beam is implemented by mechanical means using a gimbaled system to physically turn the phased array antenna. The transmission beam can also be steered electronically while keeping the phased array antenna in a fixed position. This is accomplished by electronically manipulating the phase of the transmission signal of each individual radiating element in the phased array antenna. The beam of a phased array antenna may be steered to an desired angle by applying a linearly progressive phase increment, or shift, from radiating element to radiating element. Electronic beam steering has advantages over a mechanically actuated system, greater reliability, more effective beam synthesizing and greater adaptation to hostile environments.
FIG. 1 is a simplified diagram of one row of a conventional phased array antenna 10 utilizing electronic beam steering, a complete planar phased array antenna having a number of such rows. Each radiating element 12 of the phased array antenna 10 has its own phase shifter 14. An input line 16 carrying a transmission signal is coupled to each phase shifter 14, which imparts a respective predetermined phase shift to the transmission signal as it passes through that phase shifter 14. The phase shifted transmission signals are then coupled to respective radiating elements 12 for transmission. Various types of phase shifters 14 have been developed, including switched-line phase shifters, reflection-line phase shifters and loaded-line phase shifters. The present invention is directed to an improvement over conventional switched-line phase shifters.
Present binary switched-line phase shifters are discussed in Skolnik, Radar Handbook, Second Edition (1990), pp. 7.63-7.68 and in U.S. Pat. No. 4,649,393 to Rittenbach. Present phase shifters have one or more serial connected stages, each stage having two delay lines of different length. As a transmission signal is passed through the respective phase shifter 14, it passes through each of the serially connected stages. One of the delay lines within each stage carries the transmission signal as it passes through the stages of the phase shifter. A phase shift is imparted to the transmission signal by a time delay which the transmission signal experiences as it passes through the delay line within each stage. The total phase shift is the accumulation of the individual time delays within each stage of the phase shifter.
FIG. 2 is a diagram of the conventional prior art base-2 phase shifter 14 having six serially connected stages 22a-f with two delay lines 24a,24b per stage. FIG. 2 further illustrates the phase shift that can be imparted by each of the stages 26a-f. In switched-line phase shifting, one delay line in each stage is dedicated to zero phase shift or zero time delay. For instance, in stage one delay line 24b is dedicated to zero phase shift. The zero phase shift delay lines in each stage allows the transmission signal to pass through each stage and the entire phase shifter without a phase shift, if desired. In prior art phase shifter 14 half of the delay lines are dedicated to zero phase shift. The greater percentage of delay lines dedicated to zero phase shift results in fewer lines dedicated to imparting phase shifts and a corresponding reduction in phase shift resolution.
The desired delay line within each stage is activated for carrying the transmission signal by closing the appropriate switches at the input and output of the desired delay line. The switch closing is generally controlled by a microprocessor over electrical control lines with either parallel or serial access to the switches. The switches could also be controlled by other modalities such as optical control.
The prior art switches include PIN diode switches and FET transistor switches. It is well known that switches of this type suffer from insertion loss dominated by the resistive loss of the signal line. Furthermore, switches characteristically reflect signals during transmission, causing signal distortion. As the transmission signal passes through each stage of the phase shifter, the transmission signal experiences loss and signal distortion from the switches within the stages. The greater the number of stages the greater the resulting loss and distortion.